Fail when expected files are not produced by protoc (#4287)

**What type of PR is this?**
Bug fix

**What does this PR do? Why is it needed?**
This PR fail the action if a plugin does not produce a valid Go file
that it is supposed to produce. This disallows people to add gRPC plugin
to a `go_proto_library` target that has no service definition, but
avoids the cache poisoning due to bugs in proto plugins. Because Gazelle
is able to switch between proto and grpc plugins depending on the
existence of service definition, existing repos managed by Gazelle will
not be affected.

**Which issues(s) does this PR fix?**

Fixes #3949

**Other notes for review**
Credits to @r-hang for coming up with the patch

Author: linzhp

go/tools/builders/protoc.go

@@ -20,6 +20,8 @@ import (
 	"errors"
 	"flag"
 	"fmt"
+	"go/parser"
+	"go/token"
 	"io/ioutil"
 	"log"
 	"os"
@@ -36,7 +38,7 @@ type genFileInfo struct {
 	created    bool         // Whether the file was created by protoc
 	from       *genFileInfo // The actual file protoc produced if not Path
 	unique     bool         // True if this base name is unique in expected results
-	ambiguious bool         // True if there were more than one possible outputs that matched this file
+	ambiguous bool         // True if there were more than one possible outputs that matched this file
 }
 
 func run(args []string) error {
@@ -170,8 +172,8 @@ func run(args []string) error {
 		case !copyTo.unique:
 			// not unique, no copy allowed
 		case copyTo.from != nil:
-			copyTo.ambiguious = true
-			info.ambiguious = true
+			copyTo.ambiguous = true
+			info.ambiguous = true
 		default:
 			copyTo.from = info
 			copyTo.created = true
@@ -183,15 +185,9 @@ func run(args []string) error {
 	for _, f := range files {
 		switch {
 		case f.expected && !f.created:
-			// Some plugins only create output files if the proto source files have
-			// have relevant definitions (e.g., services for grpc_gateway). Create
-			// trivial files that the compiler will ignore for missing outputs.
-			data := []byte("// +build ignore\n\npackage ignore")
-			if err := ioutil.WriteFile(abs(f.path), data, 0644); err != nil {
-				return err
-			}
-		case f.expected && f.ambiguious:
-			fmt.Fprintf(buf, "Ambiguious output %v.\n", f.path)
+			return fmt.Errorf("file %q expected from plugin %q but not created", f.path, *plugin)
+		case f.expected && f.ambiguous:
+			fmt.Fprintf(buf, "Ambiguous output %v.\n", f.path)
 		case f.from != nil:
 			data, err := ioutil.ReadFile(f.from.path)
 			if err != nil {
@@ -207,6 +203,15 @@ func run(args []string) error {
 			fmt.Fprintf(buf, "Check that the go_package option is %q.", *importpath)
 			return errors.New(buf.String())
 		}
+
+		if filepath.Ext(f.path) != ".go" {
+			continue
+		}
+		// Additionally check that created files are valid, because invalid Go file causes cache poisoning.
+		_, err := parser.ParseFile(token.NewFileSet(), f.path, nil, parser.PackageClauseOnly)
+		if err != nil {
+			return fmt.Errorf("plugin %q created an invalid Go file %q: %v", *plugin, f.path, err)
+		}
 	}
 
 	return nil